In general, satellite transmission technology uses an orbiting satellite to transmit an electromagnetic signal of very high frequency for reception by earth based antennas. Once received by the antennas, the high frequency signal is converted into a lower frequency signal so that it may be more easily manipulated by additional circuitry and/or transmitted through cables over ground. The device used to convert the transmitted signals from a higher frequency to a lower frequency is normally a Low Noise Block downconverter (LNB) and it is this device that has been improved in accordance with the invention.
In satellite transmissions of television signals, each television channel normally occupies a finite bandwidth of approximately 30 Mhz in the frequency spectrum. In order to reduce the interference between two adjacent channels, the signals for any two adjacent channels are polarized into opposite directions. For example, if the two signals are circularly polarized, one would have a polarization having clockwise direction, whereas the other one would have a counter clockwise direction; if the two signals are linearly polarized, one would have a vertical and the other a horizontal polarization. Thus, for a given number of channels, the channels having even numbers have the same polarization whereas the channels having odd numbers have the same but opposite polarization as the even channels.
A typical satellite receiver utilizes a single LNB downconverter to receive the signal from an antenna. Additionally, the receiver includes a switch for selecting a particular polarization; it cannot receive two oppositely polarized signals at once. The switch is normally placed at the receiving antenna or in the downconverter. Thus the receiver is capable of receiving signals with their polarizations.
The above-described satellite receiver, which utilizes a single LNB downconverter, has the disadvantage that it can only receive signals having the polarization at a time; it cannot be used in applications where signals having opposite polarizations are to be downconverted simultaneously. For example, in a household having two TV sets and one satellite antenna, using such a receiver would result in the two television sets being limited to play the channels whose signals having the same polarization; as a result, if one TV set is tuned to an even number channel, the other TV set is required to play only the even number channels.
A solution to this problem is to utilize two LNB downconverters, each being individually connected to a power supply and each having its own polarization switch but receiving signals from the same antenna. Since each LNB downconverter works independently of the other, two distinct transmission signals may be downconverted simultaneously through the two LNB downconverters. Consequently, if two television sets were employed at the same time, each could receive its own separate channel.
One considerable disadvantage of using a two-downconverter system in this manner is that it uses substantially more power than the single downconverter system. In a satellite receiver, because of constraints from other devices in the receiver, the dc power supply normally supplies a dc voltage that is more than twice the voltage required for a single downconverter. Thus, the voltage must be decreased through a regulator so that only the required voltage is applied to each downconverter.
For example, if the power supply provides dc voltage of 14 V, and the downconverters are designed such that they only require 6 V, then the dc voltage must be adjusted through a voltage regulator so that only 6 V is applied to each downconverter. The voltage regulator is connected to the downconverter in series; a serially connected downconverter and voltage regulator are attached to the voltage supply. In such a configuration, each downconverter draws its own current from the power supply, so that the power consumption for two would be twice what is consumed using a single downconverter.
The high power consumption of the two downconverter system increases the burden of the dc power supply. Additionally, the high current drawn by the two downconverters (in the order of 200 mA) increases the thermal strain on the electronic components in the dc power supply as well as the LNBs, and undermines their life expectancy.
Accordingly, a central object of the present invention is to provide a downconverter that is capable of simultaneously processing two oppositely polarized television signals but with low power consumption; and
A further object of the invention is to increase the life span of the dc power supply and the LNBs in the satellite receiver.